This invention relates to rocker arm lifter assemblies. More particularly, this invention relates to rocker arm lifter assemblies of a type commonly used in motorcycle engines in which the pushrod of the rocker arm lifter assembly is not coaxial with the remainder of the lifter assembly.
Conventional pushrod-operated motorcycle engines, such as those manufactured by Harley Davidson Corporation, use a valve lifter assembly comprising a housing in the form of an approximately trapezoidal flange having two hollow cylindrical sections integral with and extending through the flange. The hollow interiors of the cylindrical sections, which are inclined at a slight angle to one another, each accommodate a pushrod lifter (or tappet) the lower end of which is in contact with a cam. These tappets incorporate hydraulic lifters arranged to maintain proper valve clearances as the engine warms up. The upper end of each tappet lifter is in contact with the lower end of the pushrod, the upper end of the pushrod being in contact with the rocker arm. The pushrods are adjustable in length to allow for adjustment of valve clearances and to enable the pushrods to be inserted into and removed from the valve lifter assembly.
In this type of valve lifter assembly, each tappet has three main parts, the first of which is a hollow cylindrical body member closed at its lower end and carrying at its lower end a roller which is in contact with the cam. Within the hollow interior of this body member is accommodated the cylinder of a hydraulic lifter. The upper end of this hydraulic lifter cylinder has a radially-outwardly extending flange which abuts against the upper end of the body member to hold the hydraulic lifter cylinder in its correct position. The third and uppermost part is in three cylindrical sections. The lowest section constitutes the piston of the hydraulic lifter and slides within the cylinder. The middle section is of smaller diameter than the lower section, while the third and uppermost section is of larger diameter than the middle section and is provided with a socket with accommodates the rounded end of the pushrod. A helical spring is wrapped around the middle section, this spring being retained in position by the shoulders occurring where the middle section meets the other two sections. However, this helical spring is made slightly larger in diameter than the piston section so that when the three parts are assembled the lower end of the spring abuts the flange at the upper end of the hydraulic lifter cylinder. This type of tappet suffers from the disadvantage that the section of the third part carrying the socket has to protrude a considerable distance above the upper ends of the body member and cylinder in order to accommodate the spring, and the socket section is connected to the piston section only by the relatively thin and flexible middle section. Accordingly, since the design of the engine requires that the pushrod be inclined to the axis of the pushrod lifter, and because the piston is driven downwardly by the considerable force exerted by the valve closing spring as the valve closes, considerable flexing of the middle section occurs, resulting in an undesirable degree of vibration and noise from the engine, especially at high engine speeds. These problems of vibration and noise are exacerbated by the conventional pushrods, which are made of steel and are relatively heavy, so that there is a great deal of inertia to overcome as the pushrod oscillates up and down.
In order to reduce these vibration and noise problems, it might, prima facie, appear desirable to reduce the extent to which the socket-carrying portion of the third part protrudes above the upper ends of the body member and cylinder. However, not only would reduction of the projection of the third part above the upper ends of the body member cylinder cause difficulties in formation of a suitable spring, but the design of the conventional pushrod also makes it difficult to reduce this projection. The conventional pushrod comprises a long shaft having a rounded upper end which engages the rocker arm and a lower hexagonal section having threaded socket formed in its lower end. An adjusting member having a threaded upper section is screwed into the socket. Immediately adjacent the threaded section, the adjusting member has a section formed as a hexagonal nut and, below this nut section, a short cylindrical section the lower end of which is capped by a hemispherical section which engages the socket on the pushrod lifter. A locknut is screwed onto the threaded section of the adjusting member and can be engaged with the lower end of the shaft to lock the adjusting member and shaft in position relative to one another.
To insert this type of pushrod into position in the engine, the adjusting member is screwed into the socket in the shaft so as to reduce the length of the pushrod to a value which is less than that between the socket on the pushrod lifter and the rocker arm, so that the pushrod can be inserted between the pushrod lifter and rocker arm. The hexagonal section on the shaft and the nut section on the adjusting member are then grasped with wrenches, and the adjusting member rotated relative to the shaft until the length of the pushrod has been adjusted to the proper value. Next, while the shaft and adjusting member are being held in the correct relative position by the two wrenches already mentioned, a third wrench is applied to the locknut and the locknut rotated until it engages the lower end of the shaft, thereby locking the pushrod at its correct length.
In order that the pushrod can be adjusted to the correct length during this process, it is thus essential that the nut section on the adjusting member be accessible throughout the assembly process in order that the adjusting member can be held in the correct position relative to the shaft, and thus the correct length of the pushrod maintained, until the locknut has been engaged with the lower end of the shaft, thus locking the pushrod at its correct length. Unfortunately, final adjustment of the length of the pushrod to the correct length, which sets the clearance of the valve associated with the pushrod, must of necessity be performed with the valve in its closed position, and obviously when the valve is closed the pushrod and its associated tappet will be at their lowest positions relative to the housing. For obvious reasons, including the need to keep the relatively heavy tappet as light as possible, and the need for accurate, high-speed movement of the tappet within the cylindrical section of the housing, when the tappet is at its lowest position relative to the housing, the upper ends of the body member and cylinder lie a considerable distance below the upper end of the associated cylindrical section of the housing, and are thus inaccessible to wrenches. It is only the projection of the socket-carrying section of the third part of the tappet above the upper end of the cylindrical section of the flange that renders the nut section at the lower end of the adjusting member accessible to a wrench during final adjustment of the pushrod length. Accordingly, reduction or elimination of the projection of the socket-carrying portion of the third part above the upper ends of the body member and cylinder of the tappet cannot be achieved without considerable modification of the design of the pushrod.
Also, the cylindrico-hemispherical section at the lower end of the adjusting member, coupled with the inclination of the axis of the pushrod to the axis of the tappet, tends to cause contact of the cylindrical part of this lower end of the adjusting member with the lip of the socket at the upper end of the tappet, further increasing the noise and vibration from this type of valve lifter assembly.
Finally, lubrication of the tappets as they slide up and down within the cylindrical sections of the housing presents a problem in this type of valve lifter assembly. Because of the engine design, entry of oil into the housing has to occur at a small port located on the underside of the flange between one of the cylindrical sections and an adjacent edge of the flange. Hitherto, lubrication of the tappet in the cylindrical section adjacent the inlet port has been achieved by providing an oilway which extends up from the inlet port and then approximately horizontally through the flange into the bore of this cylindrical section. Lubrication of the bore in the other cylindrical section is achieved by a bore which passes through the wall dividing the bores in the two cylindrical sections. In practice, this frequently results in inadequate lubrication of the bore in the cylindrical section remote from the oil inlet port, since relatively little oil makes its way into the bore connecting the two cylindrical sections, and in any case this bore tends to become clogged with dirt after protracted use of the engine.
Accordingly, there is a need for modifications of the valve lifter assemblies used in such pushrod-operated motorcycle engines to overcome the aforementioned disadvantages, and this invention seeks to provide such modifications.